Thermal fuse



Sept. 25, 1956 Filed July 14, .1955

w. OTTO 2,764,025

THERMAL FUSE 2 Sheets-Sheet 1 INVENTOR. WAILTER 0770 BYKLLQ/ 20%; Km-

rromwsr Sept. 25, 1956 w. OTTO 2,76

THERMAL FUSE Filed July 14, 1955 2 Sheefs-Sheet 2 .6. "p- ,7, 76 Z 20' W 6 25 75 ll) rill/III IN V EN TOR. W141. T51? 0770 l 96 50 BY Kw am auwi United States Patent THERMAL FUSE Walter Otto, Forest Hills, N. Y.

Application July 14, 1953, Serial No. 367,864

2 Claims. (Cl. 74-2) This invention relates to an improved thermal fuse and it is an object of the invention to provide an improved device of this character which will operate as a safety device or control to indicate or prevent the overheating of various types of apparatus.

My improved invention is applicable to many different types of apparatus that are endangered by overheating, such as fired pressure vessels, for instance steam and hot water boilers, and also internal combustion engines, compressors, electric generators and motors as well as bearings and other friction producing equipment. Further uses may be found in the apparatus of the petroleum, chemical and other industries, such as heat exchangers, kilns, stills, melting and rendering vessels, reforming furnaces and the like.

Heretofore the safety and control devices available for use with such apparatus have had various disadvantages. As an example, there have been available for use with steam boilers a number of devices not fully meeting the requirements of safe operation. One of these is a safety plug inserted in the walls of boilers in the fire passage facing the fire so, that on overheating its fusible core melts, discharging a blast of steam and water into the furnace. This always sure and timely action, brought on as a result of low water or formation of boiler scale at the location of the plug, will cause damping of a coal fire to some degree, but also further empty the boiler with resultant danger thereto. Safety plugs of this type are not suitable for use with automatically fired boilers because of the damage that may be caused to gas or oil burners and other automatic firing arrangements by the steam and water discharged upon them. Furthermore, such safety plugs are not capable of stopping an automatic firing device from operating under such emergency conditions. Another type of device known as a low water cut-ofi, when applied to an automatically fired steam boiler, is intended to stop the firing device when the boiler water level recedes from its normal gage line to an undesirable degree. Since its operating parts are exposed to the water, they are subject to damage or jamming from corrosion, scale incrustation and sediment clogging, thereby rendering the device inoperative in time if not constantly attended to. A further disadvantage of this device is its inability to react directly to overheating in a boiler. It gives a signal or shuts off the firing device only when the boilers water line drops too low, thereby anticipating the result of this condition which is overheating. However, a boiler may dangerously overheat while operating with a normal water line as is the case when the water side of primary heat transfer surfaces becomes scale encrusted.

It is an object of the present invention to overcome the ditficulties and disadvantages heretofore encountered and to provide an improved thermal fuse which may serve as a safety device or control; which is relatively simple to install and service; in which the operating parts are encased or protected against damage from the water or other fluid in the apparatus being controlled; which is capable of direct contact with the parts of the protected apparatus in which the dangerous condition develops; which automatically compensates for expansion and contraction due to varying thermal conditions in order to maintain such contact; and which under predetermined thermal conditions will cause the operation of warning signals or of control switches and valves.

In the accompanying drawings Fig. 1 is a partially fragmentary, longitudinal sectional view showing an improved thermal fuse embodying my invention applied to a steam boiler;

Fig. 2 is a longitudinal sectional view of the inner end or tip portion of the fuse assembly;

Fig. 3 is a longitudinal, partially sectional view of the fuse showing a modified type of mounting bracket made from commercially available fittings and providing for a rigid mounting;

Figs. 4, 5, 6, and 7 are longitudinal sectional views of the lower or tip portions of modified type of thermal fuses embodying my invention;

Fig. 8 is a sectional view showing a modified form of working end for the thermal fuse of concave shape to provide for better contact with a boiler tube;

Fig. 9 is a longitudinal sectional view similar to Figs. 4 to 7 of a further modified form of thermal fuse;

Fig. 10 is a cross-sectional view in the direction of the arrows on the line 1010 of Fig. 9; and

Figs. 11 to 17, inclusive, are longitudinal sectional views of the inner end portions of further modified forms of thermal fuses embodying my invention.

In each form of my invention a unit with energy stored therein, such as a compressed spring, cooperated with an actuating member such as a rod or flexible cord or cable to cause the shifting of the actuating member and the actuation of a warning signal, switch or the like when the apparatus with which it is used reaches a predetermined temperature. The actuating member is restrained against movement by a fusible connection or link until the predetermined temperature is reached.

Referring to the form of apparatus shown in Figs. 1 and 2 it will be seen that my fuse assembly indicated generally by the numeral 20 is illustrated as applied to a steam boiler although as previously indicated, my invention may be used for controlling many different types of apparatus. Thus, in Fig. 1 the fuse assembly is illustrated as inserted through the outer shell or casing 21 of the boiler with the working end of the fuse in contact with the boiler tube or heat transfer surface 22.

The fuse assembly consists of a tubular casing 23 made of a suitable metal, preferably a corrosion resistant metal and closed at its working or heat exchange end 24 and open at its opposite end. Disposed inside the casing and projecting from the open end thereof is an actuating member in the form of a rod 25 connected by adjustable collar 26 (held in place by a set screw 27) and link 28 to a switch or signal 29. The rod is shown in its preset condition. When it is caused to operate due to overheating of the boiler tube the rod shifts outwardly and through collar 26 and link 28 causes the actuation of the switch vaive, or warning signal 29. if the device 29 is a switch it may, in turn, cause the operation of other warning signals or of control apparatus for the boiler.

The actuating rod may be formed of several different pieces as shown. Thus, it may have a threaded connection with head 30 secured to or formed on rod 31 which, in turn, has been cut or severed as shown at 32 from its inner tip portion 33. The particular shape or configuration of the cut 32 may vary, so long as the two sections of the rod 31 and 33 may be fitted tightly together as shown and so long as they may be separated from each other when shifted in a longitudinal direction without interference from the contacting surfaces. The partictionship and connected together at their inner ends by a longitudinally extending cut; The tip' of the rod is anchored in the working. end' 24' of the case as by being In this connection the working end is threaded therein. thick enough. to thus accommodate the tip of the rod but nevertheless should be maintained as thin as possible for the efiicient transfer of heat.

The two sections 31 and 33 of the rod are maintained in assembled relationship and the actuating member is restrained against movement under normal working temperatures of the apparatusby a fusible connection at the cutportion 32. The fusible connection is preferably formed of a metal which will melt at the desired predetermined temperature. Thus, tin' or alloys of tin or lead may be employed where it is desired to have the connection melt or fuse-at a low temperature. Lead or alloys of tin and antimony should be used where it is desired to have the connection melt or fuse at medium temperatures, and silver solder. may beemployed where it is desired to have the connection melt or fuse at higher temperatures. It should be understood, however, that thermoplastic materials may be employed for forming a connection instead. of rnetal solder. The fusible connection is located inclose proximity to the working end 24 so that the heat from theboilertube or other heat trans fer surface is readily transmitted thereto. In this connection, the tubular casing 23 or at least the working end thereof 24 should be made of material which is an efficient heat conductor.

In addition to the actuating member and fusible connection there is enclosed in the casing an energy storing unit which exerts outward force on the actuating member and this preferably takes the form of helical spring 34 which surrounds the rod'sections 31 and 33 and extends between the working end 24 and head 30. The spring is under partial compression so as to exert an outward force upon the actuating member. Under this arrangement when the fusible connection between the two sections 31 and 33 fuses or melts the spring will shift the actuating member outwardly causing the operation of the switch or warning signal 29.

The mounting bracket for'supporting the thermal fuse assembly in the boiler oroth'er apparatus provides for automatic compensation or adjustment for thermal expansion or contraction. It consists ofa fitting 35 assembled around the tubular casing 23 and having a threaded plug 36 threaded into the boiler shell or casing 21. It also has a threaded collar 37 to which the adaptor 38 is fitted.

Inside the fitting 35 at the base of collar 37 I provide a packing 39 made of suitable material such as graphited asbestos to prevent leakage from inside the boiler. The packing may be held in place by a suitable gland 40 which is accommodated inside collar 37 and adaptor 38.

The adaptor 38 has an externally threaded tubular neck 41 of reduced diameter surmounted by the spring tension cap 42. The cap is provided with an opening 43 to accommodate the actuating rod 25 as shown. Inside the cap I provide a helical spring 44 which surrounds the actuating rod and extends between the head of the cap and a follower 45 engaging the end'of tubular casing 23 and causes it to tightly engage the boiler tube or other heat transfer surface 22 regardless of variations in space due to thermal expansion and contraction.

In order to limit the stroke of the actuating member or rod 25, I may provide an adjustable stop in the form of a collar 47 secured in place by set screw 48 and assembled around the rod inside the casing so that it will engage spring follower 45 at the end of its working stroke to prevent further movement of the rod.

In using my improved thermal fuse the two sections 31 and 33 of the actuating member are first connected together. by the proper type offusible connections as previously explained and the several parts of the fuse are then assembled together and mounted in the boiler or other apparatus as shown in Fig. l. The switch or warning signal is then mounted on cap 42 as by screws 4-6, the collar 26 and link 28 are pivotally connected together, the collar adjusted to the proper position on actuating rod 35 and the set screw 27 then tightened. Should the boiler then become overheated due to insufhcient water or other causes the heat will be immediately transmitted to the fusible connection at the point 32 causing the connection to fuse or melt with the result that spring 34 will cause the actuating member to shift outwardly actuating the warning signal or switch 29. The apparatus cannot be reset by merely pressing the actuating rod 25 inwardly to its proper position. Rather, it will be necessary to disassemble the fuse and to again form a new fusible connection at the point 32. This delay is beneficial in that it permits the boiler to cool before cold water introduced which might otherwise damage the boiler.

In Fig. 3 I have shown :a modified form of mounting for the thermal fuse made from standard connection parts and which is rigid in nature so that it does not provide'for automatic compensation for thermal expansion and contraction. The fuse assembly 20 may he the same type as employed in the first form of my invention orit may be any modified form since both of the mounting brackets are suitable for use with any of the modified forms. The bracket consists of a fitting 50 having a threaded plug 51 adapted to be mounted in the shell or casing of the boiler or other apparatus to which it is applied. The fitting also has a threaded collar 52 to which cap 53 is connected. The cap has an opening to accommodate the fuse assembly. It also has a chamber 54 in the interior thereof adapted to accommodate and engage the ring 55, secured around the fuse assembly by shrinking or otherwise. In this fashion, the fuse assembly is held in fixed relative position with respect to the mounting bracket and with respect to the boiler or other apparatus to which it is applied.

The modified bracket assembly also includes a fitting 60 with thread plug 61 and threaded collar 62 similar to the plug 50 but arranged in opposite relationship near the outer end of the fuse assembly. A threaded cap 63 similar to cap 53 engages the collar 62 and has a chamber 64 accommodating the ring 65 which is secured around the fuse assembly in the same manner as ring 55. A cap '66 with an opening 67 for accommodating actuating rod 25 is fitted-over the plug 61 and engages the end of tubular casing 23. The switch or warning signal 29 is secured to cap 66 and-is actuated in the same manner as the switch in the first form of my apparatus by link 28 pivotallyconnected to collar 26' adjustably mounted on actuating rod-25.

The apparatus shown in Fig. 3 is less expensive than the apparatus shown in Fig. l and may be used in a similar manner with the exception that no provision is made for automatic compensation for differences in spacing resulting from thermal expansion and contraction. Where such differences are severe the type of bracket mounting shown in Fig. 3 should be avoided and a bracket mounting as shown in Fig. 1 should be utilized.

Referring now to Figs. 4 to 17, it will be seen that the fusible connection'may take many different forms. Thus, in Fig. 4 the fuse assembly 20 has a tubular casing 23 and an actuating member 25 disposed therein. The head portion 30 of the actuating member, however, is connected to a one-piece tube 70 which extends into a recess or opening formed in the working end 71 of the tubular casing 23. A fusible connection is then provided between the tubular member 70*and the Working end 71 as shown at 72. The fusible connection may take the form of solder or afusible metal or a thermoplastic material as previously described. A helical spring 34- is assembled around the tubular rod 70 and extends between the working end 71Eand the head 30 under compression. The

remainder of the fuse assembly is similar to that shown in the first form of my invention and it may be mounted in a boiler or other apparatus by either of the mounting brackets shown in Figs. 1 or 3. When overheating of the apparatus causes fusing or melting of the fusible connection 72 the spring 34 causes the actuating rod 25 to shift outwardly. The head portion 30 has vent holes extending therethrough as shown, so that the interior of the tube is open to the atmosphere. This prevents the mechanism from sticking due to the vacuum within the tube.

The apparatus shown in Fig. 5 is the same as that shown in Fig. 4, with the exception that a modified form of working end 71' is provided on the tubular casing. Thus, the working end 71 is of reduced external diameter and is suitable for use with journals, bearings or smaller type of apparatus. The outer surface of the working end 71' of Fig. 5 is illustrated as being fiat. However, it also may be made of tapered or conical form as shown in Fig. 7.

In the arrangement shown in Fig. 6 the inner end of the actuating member takes the form of a rod 73 which projects through flange 74 in tubular casing 23 into a chamber in the Working end 75. The chamber is filled with a fusible material such as solder, fusible metal, plastic material or the like as shown at 76 and the end of the rod embedded in the fusible material may be indented as shown at 77 so as to provide better interengagement between the rod and the fusible material.

The form of my invention shown in Fig. 7 is similar to that shown in Fig. 6 with the exception that the working end is of reduced diameter as shown at 78 and is provided with a conical tip 79.

The working end of the tubular casing may be of different shape or configuration depending upon its intended use. Thus, as previously stated, it may be flat, or it may be tapered and conical as shown in Fig. 7 or it may be of concave shape as shown at 80 in Fig. 8 so as to provide a greater contact area when it engages a boiler tube or other cylindrical article as shown at 22.

Figs. 9 and 10 show an arrangement somewhat similar to that shown in Fig. 6. However, the Working end 81 has longitudinal flanges 82 projecting into the fusible material 76 so as to increase the thermal conductivity into the fusible material. These flanges or fins may have small apertures 83 extending therethrough to increase the grip between the tubular casing 23 and the fusible material.

The arrangement shown in Fig. 11 is also somewhat similar to that shown in Fig. 6 with the exception that the working end portion 84 of the tubular casing 23 has a small internal cup 85 into which the rod 73 projects and it contains the fusible material. The interior surface of the cup may be serrated or formed with ridges or grooves to increase the grip between the fusible material and the casing. The small cup likewise serves to improve the transmission of heat to the fusible material.

The arrangement shown in Fig. 12 is similar to that shown in Fig. 11 with the exception that the cup 85' is of truncated, conical formation thereby preventing the plug of fusible material from accidentally releasing.

In the arrangement shown in Figs. 13 and 14 a spring 34' smaller in diameter than the spring 34 is positioned inside the end of the actuating member rather than exteriorly thereof. In this arrangement the inner end of the actuating member is of tubular construction as shown at 86 and the spring 34' is disposed therein. The tubular member 86 fits into the end portion 87 of the tubular casing 23 and has a fusible connection in the form of my invention shown in Fig. 13. In the form shown in Fig. 14 the working end of the casing is provided with an internal plug 88 which fits into the tubular end portion 86 and has a fusible connection therewith. The head portion 30 in the forms shown in Figs. 13 and 14 like- 6 wise have vent holes extending therethrough so that the interior of the tube is open to the atmosphere.

Fig. 15 shows a modified arrangement in which the actuating member is in the form of a flexible wire or cable rather than a rigid rod. Thus, a piston or plunger 89 is provided in the tubular casing 23 and it is formed with an aperture through which flexible wire or cable 90 extends. One end of the cable is provided with an enlarged head 91 which is embedded in the piston. The cable then extends around a guide pulley 92 journaled on shaft 93 which, in turn, is mounted in the Working end 94 of the tubular casing. A chamber is provided in the working end 94 and this is filled with fusible material 95 of the type used in the other forms of my invention. The solid fuse material holds the pulley 92 and the flexible cable captive.

The flexible cable at its outer end may connect with the linkage 28 of a warning signal or switch 29 which causes the operation thereof. Since the cable pulls inwardly while the rod shifts outwardly on the operation of the device the arrangement of the warning signal or switch is reversed in respect to that shown in Figs. 1 and 3. The remainder of the thermal fuse is assembled in the same manner and any desired type of mounting bracket such as those shown in Figs. 1 and 3 may be used. Under normal operating conditions the actuating member in the form of flexible cable or wire 90 is held against movement by the fusible material 95. When a predetermined temperature has been reached the fusible material melts or fuses releasing the pulley 92 and the flexible cable or wire for movement. The spring 34 being under compression will shift the slide 89 outwardly pulling the enlarged head 91 and the inner end of the flexible cable with it, with the result that the outer end of the flexible cable is drawn inwardly--into the tubular casing. As previously indicated, the outer end of the cable or wire is connected to a switch, valve or warning signal such as that shown at 29 so as to cause the operation thereof.

In the arrangement shown in Fig. 16 the rigid actuating rod is again employed and is connected with head 30. The head 30 is fixedly connected to Wire or cable 96. The inner end of the wire or cable has an enlarged balllike head 97 which is anchored or held by fuse metal in the working end 98 of the tubular casing. When the fusible connection melts or fuses the head is released and spring 34 causes the actuating rod to shift outwardly as in the first form of my invention.

The arrangement shown in Fig. 17 is similar to that shown in Fig. 16, with the exception that the inner end of the cable or wire 96 is not provided with an enlarged head. Instead, it is merely fitted into the working end 99 of the tubular casing 23 when soldered in place by suitable fusible material of the type used in the other forms of my invention.

It will thus be seen that I have provided an improved thermal fuse which may serve as a safety device or control; which is relatively simple to install; in which the operating parts are encased or protected against water or other fluids in the apparatus being controlled; which automatically compensates for expansion and contraction due to varying thermal conditions and which under predetermined thermal conditions will cause the operation of warning signals or of control switches.

It should be understood that modifications may be made in the illustrated and described embodiments of my invention. Thus, the fusible connection may take many different forms and the several embodiments are merely examples of varying types of connections that may be employed. In addition, the working end of the fuse assembly may be varied in shape and form so as to provide for the most efficient heat exchange, or may be externally threaded if desired so that it may be screwed directly to the apparatus with which it is used. Also, the fusible connection in any one of the modifications of my devices should be made of suitable fusible material having a melting point within the desired range.

I claim: 7

l. A thermally operated safety device for indicating and preventing the overheating of apparatus of the type having a housing and an operating portion subject to overheating comprising: a supporting fitting cooper-able to be fixedly mounted in the housing of the apparatus with which the fuse is associated; a tubular casing supported by and projecting from the fitting and having a closed working end cooperable to' be placed in heat exchange relationship with the operating portion of said apparatus; an actuating member including a flexible cable having a portion mounted within the casing for shift movement in a predetermined path between first and second predetermined positions; means actuated by said' actuating member when it shifts to said first predetermined position; means including a fusible connection Within the tubular casing adjacent the closed Working end thereof for normally holding said actuating member in the second predetermined position; and means including a helical spring located within the tubular casing and surrounding at least a portion of the flexible cable in operative relation with the actuating member when in the second predetermined position to exert force upon it to shift it towards the first predetermined position when the fusible connection is released.

2. A thermally operated safety device as set forth in claim 1 in which the fusible connection includes a guide member embedded in fusible material and the actuating member includes a shiftable plunger engaged by the spring and the flexible cable has one end connected to the shiftable plunger and extending through said spring and around the guide member and thence outwardly through the spring and plunger.

References Cited in the file of this patent UNITED STATES PATENTS 850,751 Harris Apr. 16, 1907 1,606,915 Badger Nov. 16, 1926 1,685,136 Montgomery Sept. 24, 1928 1,982,984 Durant Dec. 4, 1934 1,983,744 Dock Dec. 11, 1937 

